/***
 * ASM: a very small and fast Java bytecode manipulation framework
 * Copyright (c) 2000-2007 INRIA, France Telecom
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the copyright holders nor the names of its
 *    contributors may be used to endorse or promote products derived from
 *    this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
 * LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
 * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
 * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
 * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
 * THE POSSIBILITY OF SUCH DAMAGE.
 */
package com.elancom.framework.commons.json.asm;

/**
 * A label represents a position in the bytecode of a method. Labels are used for jump, goto, and switch instructions, and for try catch blocks. A label
 * designates the <i>instruction</i> that is just after. Note however that there can be other elements between a label and the instruction it designates (such
 * as other labels, stack map frames, line numbers, etc.).
 * 
 * @author Eric Bruneton
 */
public class Label {

	/**
	 * Indicates if the position of this label is known.
	 */
	static final int RESOLVED = 2;

	/**
	 * Field used to associate user information to a label. Warning: this field is used by the ASM tree package. In order to use it with the ASM tree package
	 * you must override the {@link com.elancom.framework.commons.json.asm.tree.MethodNode#getLabelNode} method.
	 */
	public Object info;

	int status;

	/**
	 * The line number corresponding to this label, if known.
	 */
	int line;

	/**
	 * The position of this label in the code, if known.
	 */
	int position;

	/**
	 * Number of forward references to this label, times two.
	 */
	private int referenceCount;

	/**
	 * Informations about forward references. Each forward reference is described by two consecutive integers in this array: the first one is the position of
	 * the first byte of the bytecode instruction that contains the forward reference, while the second is the position of the first byte of the forward
	 * reference itself. In fact the sign of the first integer indicates if this reference uses 2 or 4 bytes, and its absolute value gives the position of the
	 * bytecode instruction. This array is also used as a bitset to store the subroutines to which a basic block belongs. This information is needed in
	 * {@linked MethodWriter#visitMaxs}, after all forward references have been resolved. Hence the same array can be used for both purposes without problems.
	 */
	private int[] srcAndRefPositions;

	// ------------------------------------------------------------------------

	/*
	 * Fields for the control flow and data flow graph analysis algorithms (used to compute the maximum stack size or the stack map frames). A control flow
	 * graph contains one node per "basic block", and one edge per "jump" from one basic block to another. Each node (i.e., each basic block) is represented by
	 * the Label object that corresponds to the first instruction of this basic block. Each node also stores the list of its successors in the graph, as a
	 * linked list of Edge objects. The control flow analysis algorithms used to compute the maximum stack size or the stack map frames are similar and use two
	 * steps. The first step, during the visit of each instruction, builds information about the state of the local variables and the operand stack at the end
	 * of each basic block, called the "output frame", <i>relatively</i> to the frame state at the beginning of the basic block, which is called the
	 * "input frame", and which is <i>unknown</i> during this step. The second step, in {@link MethodWriter#visitMaxs}, is a fix point algorithm that computes
	 * information about the input frame of each basic block, from the input state of the first basic block (known from the method signature), and by the using
	 * the previously computed relative output frames. The algorithm used to compute the maximum stack size only computes the relative output and absolute input
	 * stack heights, while the algorithm used to compute stack map frames computes relative output frames and absolute input frames.
	 */

	/**
	 * Start of the output stack relatively to the input stack. The exact semantics of this field depends on the algorithm that is used. When only the maximum
	 * stack size is computed, this field is the number of elements in the input stack. When the stack map frames are completely computed, this field is the
	 * offset of the first output stack element relatively to the top of the input stack. This offset is always negative or null. A null offset means that the
	 * output stack must be appended to the input stack. A -n offset means that the first n output stack elements must replace the top n input stack elements,
	 * and that the other elements must be appended to the input stack.
	 */
	int inputStackTop;

	/**
	 * Maximum height reached by the output stack, relatively to the top of the input stack. This maximum is always positive or null.
	 */
	int outputStackMax;

	/**
	 * The successor of this label, in the order they are visited. This linked list does not include labels used for debug info only. If
	 * {@link ClassWriter#COMPUTE_FRAMES} option is used then, in addition, it does not contain successive labels that denote the same bytecode position (in
	 * this case only the first label appears in this list).
	 */
	Label successor;

	/**
	 * The next basic block in the basic block stack. This stack is used in the main loop of the fix point algorithm used in the second step of the control flow
	 * analysis algorithms. It is also used in {@link #visitSubroutine} to avoid using a recursive method.
	 * 
	 * @see MethodWriter#visitMaxs
	 */
	Label next;

	// ------------------------------------------------------------------------
	// Constructor
	// ------------------------------------------------------------------------

	/**
	 * Constructs a new label.
	 */
	public Label() {
	}

	// ------------------------------------------------------------------------
	// Methods to compute offsets and to manage forward references
	// ------------------------------------------------------------------------

	/**
	 * Puts a reference to this label in the bytecode of a method. If the position of the label is known, the offset is computed and written directly.
	 * Otherwise, a null offset is written and a new forward reference is declared for this label.
	 * 
	 * @param owner
	 *            the code writer that calls this method.
	 * @param out
	 *            the bytecode of the method.
	 * @param source
	 *            the position of first byte of the bytecode instruction that contains this label.
	 * @param wideOffset
	 *            <tt>true</tt> if the reference must be stored in 4 bytes, or <tt>false</tt> if it must be stored with 2 bytes.
	 * @throws IllegalArgumentException
	 *             if this label has not been created by the given code writer.
	 */
	void put(final MethodWriter owner, final ByteVector out, final int source) {
		if ((status & RESOLVED) == 0) {
			addReference(source, out.length);
			out.putShort(-1);
		} else {
			out.putShort(position - source);
		}
	}

	/**
	 * Adds a forward reference to this label. This method must be called only for a true forward reference, i.e. only if this label is not resolved yet. For
	 * backward references, the offset of the reference can be, and must be, computed and stored directly.
	 * 
	 * @param sourcePosition
	 *            the position of the referencing instruction. This position will be used to compute the offset of this forward reference.
	 * @param referencePosition
	 *            the position where the offset for this forward reference must be stored.
	 */
	private void addReference(final int sourcePosition, final int referencePosition) {
		if (srcAndRefPositions == null) {
			srcAndRefPositions = new int[6];
		}
		if (referenceCount >= srcAndRefPositions.length) {
			final int[] a = new int[srcAndRefPositions.length + 6];
			System.arraycopy(srcAndRefPositions, 0, a, 0, srcAndRefPositions.length);
			srcAndRefPositions = a;
		}
		srcAndRefPositions[referenceCount++] = sourcePosition;
		srcAndRefPositions[referenceCount++] = referencePosition;
	}

	/**
	 * Resolves all forward references to this label. This method must be called when this label is added to the bytecode of the method, i.e. when its position
	 * becomes known. This method fills in the blanks that where left in the bytecode by each forward reference previously added to this label.
	 * 
	 * @param owner
	 *            the code writer that calls this method.
	 * @param position
	 *            the position of this label in the bytecode.
	 * @param data
	 *            the bytecode of the method.
	 * @return <tt>true</tt> if a blank that was left for this label was to small to store the offset. In such a case the corresponding jump instruction is
	 *         replaced with a pseudo instruction (using unused opcodes) using an unsigned two bytes offset. These pseudo instructions will need to be replaced
	 *         with true instructions with wider offsets (4 bytes instead of 2). This is done in {@link MethodWriter#resizeInstructions}.
	 * @throws IllegalArgumentException
	 *             if this label has already been resolved, or if it has not been created by the given code writer.
	 */
	boolean resolve(final MethodWriter owner, final int position, final byte[] data) {
		boolean needUpdate = false;
		status |= RESOLVED;
		this.position = position;
		int i = 0;
		while (i < referenceCount) {
			final int source = srcAndRefPositions[i++];
			int reference = srcAndRefPositions[i++];
			int offset;
			if (source >= 0) {
				offset = position - source;
				if (offset < Short.MIN_VALUE || offset > Short.MAX_VALUE) {
					/*
					 * changes the opcode of the jump instruction, in order to be able to find it later (see resizeInstructions in MethodWriter). These
					 * temporary opcodes are similar to jump instruction opcodes, except that the 2 bytes offset is unsigned (and can therefore represent values
					 * from 0 to 65535, which is sufficient since the size of a method is limited to 65535 bytes).
					 */
					final int opcode = data[reference - 1] & 0xFF;
					if (opcode <= Opcodes.JSR) {
						// changes IFEQ ... JSR to opcodes 202 to 217
						data[reference - 1] = (byte) (opcode + 49);
					} else {
						// changes IFNULL and IFNONNULL to opcodes 218 and 219
						data[reference - 1] = (byte) (opcode + 20);
					}
					needUpdate = true;
				}
				data[reference++] = (byte) (offset >>> 8);
				data[reference] = (byte) offset;
			} else {
				offset = position + source + 1;
				data[reference++] = (byte) (offset >>> 24);
				data[reference++] = (byte) (offset >>> 16);
				data[reference++] = (byte) (offset >>> 8);
				data[reference] = (byte) offset;
			}
		}
		return needUpdate;
	}

}
